No, I'm looking for the specs of the 50 watt OT, Output Transformer.

I've got some spare transformers sitting around. Can I drop in a Hammond 1650R into my 50 watt without issues? I'm hoping for an 'upgrade'
100 watts RMS, Primary impedance 5K, 318mA Max DC per side.

Also, are there any spec sheets on Nik's 50 watt OTS OT?

I hated the mushy bottom. A set of EL34's will cure the problem of too much sag, without taking away all of they "cushyness" IMO 6L6's just sound too lose for some of these amps.

Parrallel sounds better to me. They are also more reliable. If 1 wire or speaker on the series settup goes bad you just lost all your sound. With a parrallel settup you will still get sound, you'd just need to compensate impedance at the amp, if you had lost a speaker, for example.

The name sounds familiar, but I have no experience with any of their cabs. IME as long as the cab is built decent and holds together, most of the tone comes from the speakers, things like size and material of the cab do have a small effect on tone, but as ive said, as long as it's built good it should sound good, the other stuff doesnt matter as much unless the cab was built and tuned around a specific set of speakers.

The filter is one of the easier parts, and I believe there are some schematics floating around. The OT can be pretty much anything with good headroom that can handle the wattage.

+1000

Modded, but nothing crazy. I wanted to tweek the amp to my tastes. I basically fattened it up with a lower value slope resistor, and removing the NFB from V1. For the OD I just adjusted the snubbers, V2a now has no snubber while V2b is about 880pf, I prefered the tone without the first stage, seems smoother and less "fizzy."

I also upgraded the choke and im using KT88's which sound phenomenal with this amp, IMO.

Despite these "fattening" mods I still run the treble at 3, mids at 10, with the mid boost on. And I use overwound pickups. I have a serious midrange addiction.

what do you want thicker? Clean tone? Overdrive? Both?

Some easy reversable mods I like are messing with the slope resistor (which will effect both) and the snubber caps which will effect the overdrive tone. To thicken it up i'd increase the value of the V2b snubber (one closer to the PI) to smooth out the highs a bit.

IMO the OTS amps are better suited for 2x12 settups. 4x12 are a horrible design, actually not designed at all, Jim marshall originally built it because at the time he only had 25 watt speakers but was building 100 watt amps and needed a quick fix. 4x12s tend to beam treble and have a muddy bass. Stand right infront of any 4x12 and you know what im talking about, ice pick treble. Move 2 steps to the right and now the amp sounds muddy. 2x12's tend to spread sound better which I find better suited to the OTS's characteristics. Also not a big fan of G12T75's with dumble clones, they tend to scoop the amp's incredibly sweet sounding mids.

You can mic the CGold off axis towards the center of the cab to pickup both speakers. The EVM is more efficient than the gold so the mic would have no trouble picking it up.

That said you can probably get better results with two mics, but it'll take some dialing in, which is why this option should be more for when youre in the studio. IMO

Fender has been doing this since the 60's. The "normal" and "vibrato" channels use separate but identicle preamps into the same power amp. I think a better idea would be to build a tube power amp and then just build/use whatever preamps you need.

the 3rd 12AX7 is the PI, or Phase Inverter tube.

Nik is releasing a 200 watt Dumble SSS clone which could definatly be used for bass,(Alexander Dumble actually plays bass) it was designed for lots of clean headroom.
As long as you have a proper bass cab it would easily handle it and probably sound sweet.